[Verifier] Try to fix MSVC build

Some buildbots fail with:

> C:\a\llvm-clang-x86_64-expensive-checks-win\llvm-project\llvm\lib\IR\Verifier.cpp(4352): error C2678: binary '==': no operator found which takes a left-hand operand of type 'const llvm::MDOperand' (or there is no acceptable conversion)

Possibly the explicit MDOperand to Metadata* conversion will help?